In the operation of a semiconductor chip, for example of the type packaged for operation in a computer or other larger system, it is desirable to be able to measure and monitor chip temperature while the chip is powered up and running. Such temperature monitoring can be used, for example, to identify occurrences of overheating, and to initiate the shutdown of the system so as to avoid extensive damage. It is desirable that such temperature monitoring can be performed in ranges from cryogenic to well above that of typical, ambient operating temperatures, i.e from 77 Kelvin (K.) to 400 K.
It is well known in the art to use a conventional, homojunction diode as a temperature sensitive device, the turn-on voltage of the diode being indicative of the ambient diode junction temperature. Due to carrier freeze-out effects, however, the characteristics of pn semiconductor junction diodes change dramatically at very low temperatures (i.e. &lt;100 K.), requiring that at least one side of the junction be heavily doped to provide an ohmic, metal-semiconductor contact. Such diodes require extra processing steps to manufacture, and provide less than optimal room-temperature performance.
Silicon and germanium diodes inherently have a very small variation in the range of their turn-on voltages, typically less than about 1.2 V and 0.6 V, respectively, making accurate temperature determinations difficult.
It is thus difficult to provide diode-based temperature sensing circuits having both large temperature ranges and accuracy.
U.S. Pat. No. 4,854,731 to Jenkins shows a temperature sensing circuit utilizing diodes set in polysilicon and spaced from a semiconductor element by a dielectric layer. A temperature measuring circuit is connected to the diodes, the circuit using the diodes as temperature sensitive devices, whereby to measure the temperature of the semiconductor element. Various characteristics of the diodes, such as current flow or turn-on voltage, are used to determine the temperature of the diodes and hence the semiconductor element. As noted above, diodes of the type shown in Jenkins are typically not capable of use at cryogenic temperatures.
U.S. Pat. No. 3,812,717 to Miller et al. shows a temperature sensing circuit utilizing a diode wherein the thickness of the zero bias depletion layer is more than about four times the carrier diffusion length. In an indirect bandgap material, such as silicon, this is accomplished using an intrinsic layer at the pn junction. Using direct bandgap materials such as GaAs, this is accomplished by design. The thusly formed diode is used in a current switching circuit, with the voltage drop across the diode measured to determine the temperature of the diode. This teaching by Miller et al. suffers from the complexity of the current switching circuit required to measure the temperature.
U.S. Pat. No. 4,643,589 Krause et al. shows a thermometry system employing a galium aluminum arsenide diode sensor. While the system is reported to be accurate and stable over a wide range of temperatures, it suffers from the inherent drawback of being constructed of gallium arsenide. Gallium arsenide and gallium aluminum arsenide diodes cannot be incorporated in silicon-based semiconductor chips.
It would thus be desirable to provide a temperature sensitive diode, for use in a temperature sensing circuit, which is accurate over a wide range of temperatures. Such a system would be particularly useful if the thermometry circuit was simple in implementation, and even more useful if the entire system could be integrated on a single, silicon-based chip.